1. Field of the Invention
The present invention relates to coherent signal transponders used in conjunction with pulse Doppler radars equipped for spectral analysis of the received signals. Surveillance radars and the like can be equipped with means for detailed analysis of the spectra of Doppler frequency components in the received signal. This capability is useful, for example, in separating the echos of moving objects such as vehicles and aircraft from the echos of fixed terrain features and in evaluating the velocities of such moving objects. This capability for radar signal analysis is very flexible and versatile if the apparatus affords a simultaneous measurement of many subdivisions of the Doppler frequency spectrum. Such a measurement can be performed by passing the video signal from the radar through a bank of bandpass filters having center frequencies spaced throughout the Doppler frequency range, or by performing a fast Fourier transform analysis of the radar's video signal. Functionally, such processes have a multiplicity of outputs, each of which indicates the signal strength in a certain portion of the Doppler frequency range. If the radar is observing only terrain features and vegetation, for example, only the lowest frequency terminals of the signal analyzer will show any significant outputs. If a moving object such as an aircraft or ground vehicle is observed by the radar, a signal will appear at the terminal of the signal analyzer that corresponds to the Doppler frequency generated by the velocity of this moving target.
With many forms of surveillance radars, transponders are used to either designate fixed locations on the ground or to identify moving targets of special significance. It is useful to code the signals of such beacons for identification or to transmit information from the transponder location to the radar operators. With simple pulse radars, such coding is applied by transmitting a series of pulses with a specific spacing in time between them.
2. Description of the Prior Art
In one type of prior art device, typified in U.S. Pat. No. 3,331,070, entitled "Radar Moving Target Simulator", and issued July 11, 1967, a device is constructed to receive a signal from a MTI radar and reflect back successive pulses to the MTI radar with each of the reflected successive pulses being phase shifted in time by a preselected amount to simulate a moving target. A similar type of device is disclosed in U.S. Pat. No. 3,716,856, entitled "Polarization Adaptive MTI Radar Transponder", issued Feb. 13, 1973. In both of these prior art devices the transponder or remote unit introduces a predetermined simple phase shift in the response to simulate a single signal having a given Doppler frequency shift. At the radar this return signal would appear as a moving target and could not be differentiated from actual moving targets in the area.